Based on what you wrote, and on the other good work you have already done, I think your suspicions about the thermostat possibly being the wrong part are worth investigating further.
The correct type thermostat for this engine is a simple one with a block-off plate for the engine's internal bypass return (when the tstat is open). It does not use a gasket around the perimeter of the thermostat flange; rather, it just has a simple O-ring that goes on the outside of the tstat.
Can you post a picture or part number of the thermostat you installed, or its box? What brand? And what temperature rating? For the correct type tstat, there are several tempeature ratings available, the common ones being 80C and 87C. The 80C version has proved to be the best choice for promoting stable engine temperatures especially in vehicles that are subjected to hard work.
I would start by making sure this area is right, then move on to other possibilities after confirming it.
Here is what the correct thermostat looks like, as sold by Brickwerks there in UK. Does yours look like this?
https://www.brickwerks.co.uk/lt-part...2-4d-80-c.html
Here is another view of it, below, showing the back (towards engine) side of the thermostat and the circular block-off plate it incorporates. Did the tstat you installed have this plate? Many other thermostats do not. If a thermostat is installed that fits in the main opening but lacks this plate, the result will be that the water pump is continually pulling coolant through the internal bypass, rather than the radiator, meaning the engine's cooling capacity is greatly reduced.
Let us know what you find -- it is probably something simple if it overheats this quickly. And you are certainly right to not want to drive it at all until it's able to stay cool.
